This invention relates generally to X-ray tube anode targets and, more particularly, to composite structures for X-ray tube rotating anode targets.
With increased demands being placed on the performance of X-ray tubes, manufacturers have looked for ways to increase the efficiency and/or enhance the longevity of the X-ray tube target. One approach has been to substitute a graphite material for the conventional molybdenum material used in the target body. Graphite offers the advantages of both significantly higher heat storage capacity and lower density. The increased heat storage capacity allows for sustained operation at higher temperatures, whereas the lower density allows for the use of bigger targets with less mechanical stress on the bearing materials.
Along with the advantages of the graphite targets as discussed above, there are certain problems to overcome when one chooses that material over the commonly used refractory metal. First, it is more difficult to attach the graphite body to the rotatable stem of the X-ray tube than it is to attach a metal disc. Secondly, when a focal track is applied directly to a graphite substrate, the rate of heat transfer from the focal track to the substrate is slower than when the focal track is attached to a metal substrate. Under certain operating conditions, this can cause an overheating of the focal track and resultant damage to the target.
A known approach for obtaining the advantages of each of the commonly used materials, i.e., refractory metal and graphite, is to use a combination of the two in a so-called composite substrate structure. This structure is commonly characterized by the use of a refractory metal disc which is attached to the stem and which has affixed to its front side an annular focal track. Attached to its rear side, in concentric relationship to the stem, is a graphite disc which is, in effect, piggybacked to the refractory metal disc. Such a combination provides for (a) an easy attachment of the metal disc to the stem, (b) a satisfactory heat flow path from the focal track to the metal disc and then to the graphite disc, and (c) the increased heat storage capacity along with the low density characteristics of the graphite disc.
With a composite target, one of the main concerns is that of attaching the graphite portion to the refractory metal portion in a satisfactory manner. In addition to the obvious strength requirements, which are substantial when considering the rotational speeds of up to 10,000 RPM, relatively high operating temperatures on the order of 1,200.degree. C. must also be accomodated. In addition, the metal and graphite elements must be adequately joined so as to provide for the maximum transfer of heat from the metal portion to the graphite portion. For example, it has been found that if there are air pockets between the two portions, the heat transfer characteristics will be inadequate in those sections.
A common method for joining the graphite portion to the metal portion is that of furnace or induction brazing with the use of an intermediate metal. Zirconium has been commonly used for that purpose because of its excellent flow and wetting characteristics. A problem that arises with the use of zirconium, however, is the formation of carbides at the interface between the zirconium and the graphite. Since the carbides tend to embrittle the joint, the strength of a joint is inversely related to both the amount of carbide formed and the thickness of the carbide layer. The amount of the carbide formed depends on the thermal history of the component during both the manufacturing and the operational phases thereof, neither of which can be adequately controlled so as to ensure that the undesirable carbides are not formed.
Other materials have been found useful in attaching the graphite portion to the metal portion of the target. A group of such materials that has been particularly suitable for such an attachment are those discussed in U.S. Pat. No. 4,145,632, issued on Mar. 20, 1979 and assigned to the assignee of the present invention. Those materials, and platinum in particular, were found to have a significant advantage over the zirconium material because of their relative insusceptibility to forming a carbide at the graphite platinum interface.
In the composite target structure, the metal portion is generally formed of a molybdenum alloy commonly known as TZM. While TZM is the preferred material in this application, MT104 can be substituted for TZM. This alloy, in addition to molybdenum, contains about 0.5% titanium, 0.07% zirconium and 0.05% carbon. In constructing a target of TZM or MT104 and graphite using a platinum bond, it has been found that when the platinum is molten, the carbon from the graphite portion diffuses through and forms a molybdenum carbide (Mo.sub.2 C) layer between the platinum and the TZM. In addition to forming a weaker bond, the Mo.sub.2 C layer has also been found to deteriorate with thermal cycling of the target structure. Thus, there have been found some disadvantages in using only platinum to braze graphite to TZM.
It is therefore an object of the present invention to provide an improved composite X-ray target with a brazed interconnection having good strength and heat transfer characteristics.
Another object of the present invention is to provide a method and brazing material for minimizing carbide formations in braze joints between a graphite element and a metal element.
These objects and other features and advantages will become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.